I. Field of the Invention
The present invention relates to communication systems in general and, in particular, to improving the transmission of information signals in a communications system.
II. Description of the Related Art
CDMA communication systems are very sensitive to peak transmit power and are generally limited by interference related to transmit power levels. One interference related limitation is the so called “Near-Far Problem”. In this problem as transmit power increases during a transmission it causes more interference in other channels. To deal with this additional interference the other channels must increase their own transmit power. The increase in transmit power by the other channels in turn generates more interference for all the channels. This avalanche effect occurs until the system is stabilized and all the channels are satisfied. Therefore, in order to maximize the capacity of such a system it is desirable that each user transmit only the minimum power necessary to achieve a required quality of service. Another problem that can degrade the performance of other links in a transmission system is a waveform that contains a discontinuous power pattern. This problem compounds the Near-Far Problem.
Transmit power amplifiers provide another area where interference can limit the capacity of CDMA communication systems. The maximum output power of transmit power amplifiers is determined by a number of design parameters including power dissipation and unwanted emissions. Unwanted emissions are those that are outside the bandwidth of the input signal. Most of the unwanted emissions occur due to intermodulation within the power amplifier. Intermodulation is caused by high transmit power levels that drive the amplifier into a nonlinear region.
Unwanted emissions are often limited by regulatory bodies, such as the FCC. Industry standards may also set limits on unwanted emissions in order to avoid interference with the same or another system. To maintain unwanted emissions within the desired limits, the output power of the transmit power amplifier is selected so that the probability of exceeding the emission limits is very small. When a waveform having a nonlinear envelope is amplified, the maximum output is determined by the portion of the waveform that has the highest power level. Additionally, if the requested output power exceeds the maximum permitted output power, a transmitter can limit the output power to the maximum permitted level in order to keep the unwanted emissions within the prescribed limits.
Referring now to FIG. 1, there is shown graphical representation 10 of transmission waveforms 12, 18. Transmission waveform 12 is formed of waveform portions 14, 16 having differing power levels. The transmit power level limitation of the amplifier is reached by portion 14 rather than by portion 16 because portion 14 has the highest instantaneous power. In contrast, transmission waveform 18 has a constant envelope. Transmitting at the maximum power permits higher energy transmission, as illustrated by the areas under transmission waveforms 12, 18. In order to maximize the total transmit energy over a period of time it is therefore desirable that the signal applied to the transmitter have a peak to average power ratio as close to one as possible. Furthermore, in addition to preventing the peak transmit power problems, a constant power level reduces self interference that can result from fast changes of the loading in the power amplifier.
For example, FIG. 2 shows a plurality of transmission waveforms 20a–n. The number n of transmission waveforms 20a–n can be very large. For example, n can commonly have a value of two hundred or more in CDMA communication systems. Transmission signal 20a–n is formed of pilot portions 22, control portions 24, voice portions 26, and data portions 28. Pilot portions 22 of transmission signals 20a–n always have a high power level. By definition, in order to serve as a pilot signal, portion portions 22 must always be high. Data portions 28 are usually relatively high because it is a very highly utilized time slot. Voice portions 26, on the other hand, are typically low because voice signals have many unused periods. Total power waveform 30 represents the total power of transmission waveforms 20a–n summed together. Because pilot portions 22 and data portions 28 are at high levels within transmission waveforms 20a–n, the corresponding portions 32, 36 of total power waveform 30 are high. Because voice portions 26 vary and are usually low, portion 34 of total power waveform 30 can vary from close to zero to an intermediate level 34.